The present invention relates to a circuit board with which heat dissipation is improved by means of a mixture of a resin and an inorganic filler. Particularly, the present invention relates to a resin substrate having a high heat dissipation (i.e. a thermally conductive substrate) which is used for mounting power electronics devices. More particularly, the present invention relates to a thermally conductive substrate and a lead frame which comprises a thermally conductive resin sheet member and which is used for the production of the thermally conductive substrate, and production processes of such substrate and such lead frame.